The present invention relates in general to filtration modules, and in particular to a new and useful method and apparatus which utilizes a curved fluid channel constructed to maximize the formation of vortices at the solution-membrane interface, for improving the filtration effect.
Currently, most modular designs for pressure-driven membrane processes, such as reverse osmosis, ultrafiltration and microfiltration are based on maximizing membrane area per unit volume and on the handling convenience of the module. Except for rotating filter designs, no module is currently designed specifically for the purpose of minimizing concentration polarization (CP) and fouling of the membrane. Many methods exist for reducing CP and fouling, including chemical modification of the membrane surface and physical methods such as scouring. Hydrodynamic methods are also known which rely on eddies during turbulent flow, or induced flow instabilities. Such instabilities can be created by introducing inserts into the flow path. Unstable flow across membranes have also been utilized to reduce solute build-up at the solution-membrane interface, by one of the co-inventors of the present application. See Belfort, G., "Fluid mechanics in membrane filtration: recent developments", J. Membrane Sci., 40, 123-147 (1989).
Different types of instabilities have been used including vortices and instabilities resulting from rough membrane surfaces, flow pulsations and oscillating membrane surfaces. One of the most successful depolarizing methods has used Taylor vortices established in a rotating annular filter module. The main limitations of this design are the difficultly in scaling-up membrane area and high energy consumption. Vortices have also been produced in membrane-lined channels by frequently reversing turbulent flow (at 8 Hz) in a corrugated channel. See, Stairmand, J. W. and Bellhouse, B. J., "Mass transfer in a pulsating turbulent flow with deposition into furrowed walls," Int. Heat Mass Transfer, 27, 1405 (1985). This has also been done by forcing the fluid to flow around in a spiral half-cylinder channel over a flat membrane. See PCT patent application WO 90/09229 of Aug. 23, 1990 to Winzeler. Both of these approaches show increased performance in the presence of vortices, but each has experienced some difficulties including scale-up and sealing problems.
The use of spacers in a spiral wound unit to induce mixing has been widely recommended. Unfortunately, at low axial Reynolds numbers typically used in Poiseuille flow for most spiral wound units, this approach has been largely ineffective. See the spiral flow filters of Toray Industries, Inc., disclosed, for example, in their brochure entitled Romembra Toray Reverse Osmosis Elements.